1. Field of the Invention
The present invention relates to gas turbine generators and, more specifically, to a unique control system for multiple mode regulation based on output priorities to optimize turbine generator operation.
2. Description of the Background
Collectively, turbines are used to drive over 95% of the electrical generating capacity in the world. Turbines may be driven by a variety of fluids. Commonly used fluids for driving turbines include steam raised in fossil fuel-fired boilers and hot air or combustion products.
In a synchronous generator plant, the turbine drives a synchronous generator that consists of a three phase stator winding and a wound rotor which carries DC excitation current to produce a rotating magnetic field. If the generator is isolated from the power grid, then the excitation current controls the voltage generated in the stator windings.
If the generator is connected to a power grid, the excitation current is used to regulate the power factor or reactive power generated. The grid voltage is a function of the net excitation of all the connected generators. A single generating unit may have little ability to influence grid voltage. In such cases, increasing the excitation above that required for matching the generator voltage to the grid voltage produces a lagging power factor. Decreasing the excitation below that required for voltage matching produces a leading power factor.
At least two general types of exciters are commonly used to provide excitation current for the rotor. Older rotating exciters consist of a DC generator connected directly to the main rotor for rotation with it. Commutation brushes may be used to supply DC current to the main rotor slip rings. Later rotating exciters are of an alternator style employing rectifiers. A voltage regulator is used with the rotating exciter to supply the exciter field current and thereby control the generator output.
Static exciters include high output SCR rectifiers that provide all the excitation required by the main rotor. These SCR rectifiers are supplied from a step down transformer on the output of the main generator. In this case, the voltage regulator signal determines the firing time of the SCR's and thereby controls the rotor excitation current.
The voltage regulator controls the generator excitation to achieve the voltage, MVAR's (mega-volt-amperes of reactive power), or PF (power factor) desired by the operator. In the past, voltage regulators may be provided with options to control voltage, PF, and MVAR's and to limit minimum and maximum excitation to protect both the exciter and generator from damage.
For power management purposes it is often desirable to over-excite the generator to produce MVAR's required by local loads. Thus, the reactive component of the local load is generated locally and does not have to be supplied through long power transmission lines, with high losses, from a remotely located main generating plant.
Substantial problems arise in generating power locally with respect to local generation of reactive loads. For instance, low power factors used to optimize MVAR output tend to cause overloading of the generators. This is because MVAR production creates large reactive currents which may damage stator or rotor windings even though MVAR production does not consume horsepower. Thus, the turbine may overload the generator relatively easily when operating in this mode. In a peaking turbine mode, it is often desirable to generate maximum MVAR's up to the limit of the generator. There is, at the same time, substantial economic incentive to utilize full gas turbine output because at this time the turbine operates at its highest efficiency. Load changes may also require significant changes in operation modes to optimize combinations of real and reactive power output. Generator capabilities may differ from anticipated ranges of operation for the generator due to many factors such as cooling efficiency, ventilation, and the like. Generator and turbine capacity will also vary significantly depending on ambient temperature conditions. For instance, if ambient temperatures are very low, increased turbine capability at low temperatures may result in overloading the generator when the gas turbine is operated for peak loading.
Thus, there has been a long felt need to economically solve the problems of efficient generation of power including those associated with locally generated reactive power output. Those persons skilled in the art will appreciate the present invention that significantly alleviates these and other problems.